This invention relates generally to foamed materials, preferably foamed plastic materials, and to techniques for making and using such materials, and, more particularly, to the use of supercritical fluids for producing supermicrocellular foamed materials which can achieve a relatively wide range of material densities and a large number of extremely small voids or cells per unit volume therein.
Techniques for making conventional foamed materials, such as foamed polymer plastic materials, have been well known for many years. Standard techniques for such purpose normally use chemical or physical blowing agents. The use of chemical agents is described, for example, by Lacallade in the test, xe2x80x9cPlastics Engineering,xe2x80x9d Vol. 32, June 1976 which discusses various chemical blowing agents, which agents are generally low molecular weight organic compound which decompose at a critical temperature and release a gas (or gases) such as nitrogen, carbon dioxide, or carbon monoxide. Techniques using physical agents include the introduction of a gas as a component of a polymer charge or the introduction of gases under pressure into molten polymer. Injection of a gas into a flowing stream of molten plastic is described, for example, in U.S. Pat. No. 3,796,779 issued to Greenberg on Mar. 12, 1976. Such earlier used and standard foaming processes produce voids or cells within the plastic materials which are relatively large, e.g., on the order of 100 microns, or greater, as well as relatively wide ranges of void fraction percentages e.g., from 20%-90% of the parent material. The number of voids per unit volume is relatively low and often there is a generally non-uniform distribution of such cells throughout the foamed material. Such materials tend to have relatively low mechanical strengths and toughness and there is an ability to control the dielectric constant thereof.
In order to improve the mechanical properties of such standard cellular foamed materials, a microcellular process was developed for manufacturing foamed plastics having greater cell densities and smaller cell sizes. Such a process is described, for example, in U.S. Pat. No. 4,473,665 issued on Sep. 25, 1985 to J. E. Martini-Vredensky et al. The improved technique provides for presaturating the plastic material to be processed with a uniform concentration of a gas under pressure and the provision of a sudden induction of thermodynamic instability in order to nucleate a large number of cells. For example, the material is presaturated with the gas and maintained under pressure at its glass transition temperature. The material is suddenly exposed to a low pressure to nucleate cells and promote cell growth to a desired size, depending on the desired final density, thereby producing a foamed material having microcellular voids, or cells, therein. The material is then quickly further cooled, or quenched, to maintain the microcellular structure.
Such a technique tends to increase the cell density, i.e., the number of cells per unit volume of the parent material, and to produce much smaller cell sizes than those in standard cellular structures. The microcellular process described tends to provide cell sizes that are generally smaller than the critical sizes of flaws that preexist in polymers so that the densities and the mechanical properties of the materials involved can be controlled without sacrificing the mechanical properties of some polymers, such as the mechanical strength and toughness of the polymer. The resulting microcellular foamed materials that are produced, using various thermoplastics and thermosetting plastics, tend to have average cell sizes in the range of 3 to 10 microns, with void fractions of up to 50% of the total volume and maximum cell densities of about one billion (109) voids per cubic centimeter of the parent material.
Further work in producing microcellular foamed plastic material is described in U.S. Pat. No. 4,761,256 issued on Aug. 2, 1988 to Hardenbrook et al. As set forth therein, a web of plastic material is impregnated with an intert gas and the gas is diffused out of the web in a controlled manner. The web is reheated at a foaming station to induce foaming, the temperature and duration of the foaming process being controlled prior to the generation of the web to produce the desired characteristics. The process is designed to provide for production of foamed plastic web materials in a continuous manner. The cell sizes in the foamed material appear to lie within a range from 2 to 9 microns in diameter.
It is desirable to obtain improved foamed materials which will provide even smaller cell sizes, e.g., 1.0 micron or less, and much higher cell densities as high as several thousand trillions of voids per cubic centimeter, i.e., on the order of 1015, or so, voids per cubic centimeter of the parent material, for example. Such materials should also have a capability of providing a wide range of void fraction percentages from very high void fractions (low material densities) up to 90%, or more, to very low void fractions (high material densities) down to 20%, or less.
Further, it is desirable to be able to produce microcellular plastics at or near ambient temperature, so as to eliminate the need to heat the plastic during the process thereby simplifying the manufacturing process. Moreover, it is further desirable to increase the speed at which a fluid is dissolved in a polymer so that the overall time of the foaming process can be significantly reduced so as to increase the rate of production of the foamed material.
No processes used or proposed for use to date have been able to provide foamed materials having such extremely small cell sizes, such extremely high cell densities and such a wide range of material densities that provide improved material characteristics. Nor have techniques been proposed to obtain such materials at ambient temperature and at increased production rates.
In accordance with the invention, supermicrocellular foamed materials are formed by using supercritical fluids, i.e., gases in their supercritical state, which supercritical fluids are supplied to the materials to be foamed. The supercritical fluid is used as the foaming agent in a parent material, preferably, for example, in a polyester plastic material. A relatively high density supercritical fluid made at a relatively low temperature and a relatively high pressure is used to saturate the polymer without the need to raise the saturation temperature of the process to the melting point of the polymer.
While the mechanism for achieving saturation is not fully understood in detail, it is believed that the supercritical fluid (as a solute) is initially dissolved in the polymer material (as a solvent) until the concentration percentage of supercritical fluid in the polymer reaches a reasonable level, e.g., perhaps about 10% to 40%. At some percentage level then, it is believed that supercritical fluid then tends to act as a solvent and the polymer tends to act as a solute. However, whether the supercritical fluid and polymer act as solvents or solutes during the process, at some time following the introduction of supercritical fluid into the polymer, an effectively saturated solution of the fluid and the polymer is produced. Although the aforesaid description is believed to be a reasonable theoretical explanation of what occurs during the process involved, the invention is not be construed as requiring that such specific process necessarily occurs in the manner so described.
When the fluid/polymer solution contains a sufficient amount of supercritical fluid therein at a suitably selected temperature and pressure, the temperature and/or pressure of the fluid/polymer system is rapidly changed to induce a thermodynamic instability and a foamed polymer is produced. The resulting foamed material can achieve a cell density of several hundred trillions of voids per cubic centimeter and average void or cell sizes of less than 1.0 micron, in some cases less than 0.1 micron. Moreover, in accordance with the invention, the foaming of such materials can in some cases be achieved at ambient (room) temperature conditions.